Described below is a method for operating a radio communication in a multi-radio link communications system, an apparatus of a multi-radio link communications system performing such a method, and a subscriber station performing such a method.
In order to increase the resource efficiency and quality of service when transmitting data via radio links, multi-radio link communications systems are being developed in which a single subscriber station can access different resources simultaneously in parallel by one or more suitable interfaces. In particular, the intention is to enable access to resources of radio communications systems which are technologically different in kind, such that the subscriber station can operate for example in parallel both a mobile radio communications link and a radio link to a local area network. In particular, in this case a subdivision of a sequence of mutually associated data, for example of a common service, is intended to be able to be transmitted in subdivided fashion via the different resources.
In this case, the data are subdivided at the MAC or IP level (MAC: Media Access Control, IP: Internet Protocol), wherein MAC forms an interface between a logical link control and the physical layer of a network at a node and is configured differently for different physical media. In particular, the metrics resulting from subdivision into radio channels, quality, data flow and operator requirements differ. Consequently, multi-radio link communications systems afford a diversity via different resources which enable radio accesses.
In particular, three forms of transmission are planned. The first form is in a so-called switched MRTD (Multi-Radio Transmission Diversity), wherein packets of a data flow are transmitted sequentially and alternatively via only one resource which is available for MRTD. In accordance with a second form, a so-called parallel MRTD, the packets of a data flow are transmitted simultaneously via a plurality of available resources, wherein each packet is transmitted only via a single one of the resources. In accordance with the third form, a parallel MRTD with redundancy takes place, wherein the packets of a data flow are transmitted simultaneously via the resources which are available for MRTD, and wherein a copy of each packet is additionally transmitted in parallel via all resources selected for MRTD.
So-called multi-homing is generally known from J. Luo et al., “Affecting Factors for Joined Radio Resource Management and a Realisation in a Reconfigurable Radio System”, WWRF 13th Meeting, Jeju-Island, Korea, Mar. 2-3, 2005, and J. Luo, R. Mukerjee, M. Dillinger, E. Mohyeldin and E. Schulz, “Investigation on Radio Resource Scheduling in WLAN coupled with 3G-Cellular Network”, IEEE Communication Magazine, June 2003. Further principles are described in J. Luo etc., “Gain Analysis of Joined Radio Resource Management for Reconfigurable Terminals”, Multiradio Multimedia Communications (MMC 2003), Dortmund, Germany, Feb. 26-27, 2003, and Alex C. Snoeren, “Adaptive Inverse Multiplexing for Wide Area Wireless Networks”, Proc. of IEEE GlobeCom, Rio de Janeiro, December 1999.
Present-day solutions for data transmission are costly with regard to power consumption and signaling complexity, however, in the case of such a system and access method. A subscriber station disadvantageously scans all of the available resources in the form of the different radio access possibilities and their actual sub-resources available within such resources, which necessitates a great deal of power and signaling complexity. In the case of, for example, a multi-radio link access network having two resources, e.g. GPRS (General Packet Radio Service) at 128 Kbps and WLAN (Wireless Local Area Network/radio-based local area network) at 37 Mbps effective net data rate, and with a mobile subscriber station which supports both resources, each data packet is transmitted alternatively via one resource of the two resources if switched MRTD is selected. If the intention is to carry out a file download from the internet to the mobile subscriber station, the advantage with regard to a performance gain through the use of GPRS in addition to WLAN is negligible, even if the costs of the switched MRTD, for example as a result of a switching delay, are negligible.
Whenever available resources of a subscriber station offer different characteristics, for example 128 Kbps on a GPRS channel and 30 Mbps on a WLAN channel, there is the risk of the performance being impaired. The greater the difference in data rates between the resources used, the smaller the advantage of the MRTD, which also has resources having a low data rate, with regard to the total data rate.
In accordance with a second disadvantageous aspect, there is an increasing probability of a buffer overflow at the receiving subscriber station in the case of an increasingly larger data rate difference between the resources used, on account of a delay effect caused by the slow resource, if MAC segments/IP packets are re-requested in order to obtain them in the correct order at the receiver end. What is additionally disadvantageous is that a radio network subjected to a high degree of loading leads to a poor resource link on account of interference or a high dropout/blocking rate, which can be observed on the network or transport layer in the case of so-called ARMH (Adaptive Radio Multi-Homing) or IP layer multi-homing.